1. Field of the Invention
The present invention relates to a process for producing an organic acid ester of ascorbic acid or erythorbic acid by utilizing a transesterification by an enzyme.
2. Discussion of the Background
Ascorbic acid and erythorbic acid are widely used as an additive for foodstuffs and cosmetics to prevent their oxidation, because ascorbic acid and erythorbic acid have a strong reducing ability.
Nevertheless, since ascorbic acid and erythorbic acid can not easily dissolve in oils, they are converted into their organic acid ester with a high oil solubility, such as palmitate, myristate or stearate, to inhibit the oxidation of oily foodstuffs, such as nuts, potato chips, mayonnaise, margarine, fried snacks and so on.
Moreover, some salts of organic acid esters of ascorbic acid are useful as a surface active agent for foodstuffs, and anti-browning agent for fruits and flowers.
A process for preparing ascorbic acid-6-palmitic acid ester has been disclosed in Japanese Patent Application Laid-open No. 54-88261. In this process, hydrogen fluoride is used as a solvent and as a catalyst.
Also, Japanese Patent Application Laid-open No. 59-170085 has disclosed a process for preparing a fatty acid ester of ascorbic acid in which a sulfuric acid having a concentration of higher than 96% is used as a solvent and as a catalyst.
In these processes, a strongly corrosive acid, such as hydrogen fluoride or sulfuric acid, is used. Therefore, the processes have such disadvantages that apparatus and vessels must be highly resistant to corrosion, that the handling of the reaction mixture is difficult, and that a large amount of an alkali is required for the waste water.
To overcome the disadvantages of the conventional processes, a process for the preparation of an ester by reacting ascorbic acid or erythorbic acid with a carboxylic acid or its ester in an organic solvent in the presence of an enzyme as a catalyst has been proposed in Japanese Patent Application No. 27069/1990 (Japanese Patent Application Laid-open No. 3-117495, European Patent-A2 0 401 704) by one of the present inventors and others. This process has an advantage that the reaction can be carried out under milder conditions with high selectivity than those of the conventional processes and that conventional plants may be used. Nevertheless, since the ester synthesis according to this process is essentially equilibrium, the concentration of the ester of ascorbic acid or erythorbic acid in the reaction mixture is only 2 or 3% due to the formation of water. Therefore, the productivity is not satisfactory.